Differentiating Lewis Acids/Bases

[Cassidy Kohlenberger 3D]

I have read through the textbook and I'm still a little confused on how to determine whether a compound is a Lewis acid or Lewis base. I know a Lewis acid is an electron acceptor and a Lewis base is an electron donor, but I'm not quite sure how to look at a compound and determine this. Is it based on charge? If the central atom's octet is full? Lone pairs? Can someone explain...

[Aayush Patel 3B]

If you were to draw the Lewis structure out for NH3 (Ammonia), you would see that there is a lone pair on the nitrogen atom. Since Lewis bases are electron donors, this lone pair would be donated for another atom to bind to it. For example, when NH3 and water react, an H+ binds to the NH3 to become NH4+. Another example is if CO2 reacted with water, the C atom accepts an electron pair from the O from the water and a proton moves from an H2O oxygen atom to a CO2 oxygen atom.